1. Field of the Invention
The present invention relates to an optical disk drive for optically reading and/or writing data from/on a data storage medium such as an optical disk and more particularly relates to a technique of sensing the tilt of an optical disk.
2. Description of the Related Art
The storage densities of optical disks are further increasing these days. To catch up with this trend, the beam spot of a light beam for reading and/or writing data from/on such a high-density disk needs to have a further decreased size. For that purpose, an objective lens with a large numerical aperture (NA) is used more and more often. However, if the optical disk has a significant tilt due to warp, an influence of a coma aberration will become too large to ignore and the optical disk drive cannot properly perform read and write operations. To avoid such aberrations, the optical disk drive needs to be provided with a mechanism for sensing the tilt angle of the data storage layer of the optical disk with respect to the optical axis of the objective lens and correcting that tilt angle such that the optical axis of the objective lens crosses the data storage layer of the optical disk at right angles.
FIG. 15 schematically shows a configuration for a conventional optical disk drive as disclosed in Japanese Laid-Open Publication No. 2003-45058.
As shown in FIG. 15, the conventional optical disk drive includes a light source 101, a beam splitter 103, an objective lens 104, a detection optical system 106, a photodetector 107, and a tilt detector 108. The tilt detector 108 includes a signal calculator 109, an amplifier 110 and a differential amplifier 111.
The light emitted from the light source 101 and having an optical axis 102 is transmitted through the beam splitter 103 and then focused by the objective lens 104 onto the data storage layer of a given optical disk 105. Next, the light is reflected from the optical disk 105, transmitted through the objective lens 104 and then reflected by the beam splitter 103 toward the detection optical system 106. Thereafter, the light is transmitted through the detection optical system 106 and then incident onto the photodetector 107.
FIG. 16 schematically illustrates the photosensitive areas of the photodetector 107 and the cross section (i.e., beam spot 112) of the incident light beam. The photodetector 107 includes photosensitive areas 107a through 107f, which are irradiated with the light beam that has been reflected from the optical disk 105. The beam spot 112 of the light beam has an area 112a including only the zero-order component of the light that has been diffracted by a track groove of the optical disk 105, an area 112b in which the zero-order and +first-order components thereof are superposed one upon the other, and an area 112c in which the zero-order and −first-order components thereof are superposed one upon the other.
As shown in FIG. 16, the light beam with the beam spot 112 is detected by the six photosensitive areas 107a through 107f separately. The signals obtained from the photosensitive areas 107a through 107f are input to the signal calculator 109, which calculates differential signals PP1 and PP2 by the following equations:PP1=107a+107b−(107c+107d)PP2=107e−107f where the reference numerals 107a through 107f represent the signals obtained from the respective photosensitive areas 107a through 107f. 
In the tilt detector 108, the amplifier 110 multiplies the signal PP1 by k. The differential amplifier 111 calculates the difference between the signal PP1 that has been multiplied by k and the signal PP2 and outputs the difference as a tilt error signal TILT. That is to say, the tilt error signal TILT is given by:TILT=PP2−k*PP1where * is an operator of multiplication. The constant k is determined so as to correct the offset of the signal PP2, which is caused by the shift of the optical axis of the objective lens 104 from the optical axis 102 of the optical system, by the signal PP1. Thus, the tilt error signal TILT is a signal obtained by correcting the offset that has been caused by the shift of the optical axis of the objective lens 104.
If the optical disk 105 has tilted in its radial direction with respect to the optical axis 102 of the optical system, the coma aberration, produced while the light is passing through the transparent substrate of the optical disk, mainly deforms the wavefronts of parts of the light beam corresponding to the areas 112b and 112c in which the zero-order and ±first-order components of the light diffracted by the tracks are superposed one upon the other. The wavefronts are deformed differently in the respective photosensitive areas 107a through 107f. Accordingly, the signals PP1 and PP2 are subject to different types of modulation. The difference in modulation is affected by the tilt of the disk and expresses itself in the tilt error signal TILT. Thus, by detecting the tilt error signal TILT while the beam spot is following the track, the tilt of the optical disk can be detected without being affected by the shift of the objective lens so easily.
However, the present inventors discovered that when the tilt of an optical disk, on which a recorded track and an unrecorded track had different reflectances, was detected by the technique disclosed in Japanese Laid-Open Publication No. 2003-45058, the optical disk tilt detection signal was easily affected by how much the light beam was out of focus with the optical disk (which will also be referred to herein as “the degree of defocusing”) in a boundary between the recorded and unrecorded tracks. This problem will be described in further detail below.
FIG. 17A is a schematic cross-sectional view of tracks provided on the data storage layer of an optical disk. Among the three tracks illustrated in FIG. 17A, information has already been written on only the hatched track on the left-hand side. The recorded track has a lower reflectance than the other unrecorded tracks. FIG. 17B shows the waveforms of a tilt error signal TILT, which were obtained by simulating a situation where a beam spot was crossing the tracks arranged as shown in FIG. 17A, i.e., such that every third track was recorded. The following is the conditions for the simulations:
(Condition 1)
    Waveform of light source: 405 nm;    NA of objective lens: 0.85;Thickness of transparent substrate of optical disk:    100 μm    Track pitch: 0.32 μm;    Track groove width: 0.18 μm;    Track depth: 1/12 wavelength;    Reflectance of recorded track: 0.5;    Reflectance of unrecorded track: 1; and    Tilt of optical disk: NO.
Also, the ratio of the width of the photosensitive areas 107e and 10f for detecting the signal PP2 as measured in the track direction to the beam spot diameter was supposed to be 0.35, and the constant k was supposed to be 0.75, which is defined to correct the offset caused by the shift of the optical axis of the objective lens as described above.
In FIG. 17B, the abscissa represents the location of the beam spot as measured in the radial direction of the optical disk with respect to the center of the central track shown in FIG. 17A as the origin. More specifically, the respective centers of the three tracks shown in FIG. 17A were located at 0 μm and ±0.32 μm as shown in FIG. 17B. Also, in FIG. 17B, the three curves represent the waveform of the tilt error signal TILT in a situation where the light beam was exactly focused on these tracks (i.e., with no defocusing, DF=0 μm), and the waveforms of the tilt error signals TILT in situations where the focal point deviated by ±0.2 μm (i.e., DF=±0.2 μm). As shown in FIG. 17B, the respective levels of the tilt error signals TILT were the same at the beam spot location of −0.32 μm no matter whether the light beam was in focus or not, while those signal levels changed according to the focusing state at the origin and at the beam spot location of +0.32 μm. The results shown in FIG. 17B were obtained by simulating the situation where the disk had no tilt. However, the level of the tilt error signal TILT still changed according to the focusing state. For that reason, the conventional tilt error signal TILT to be detected while the beam spot is following the center or right-hand-side one of the three tracks shown in FIG. 17A changes its level according to the degree of defocusing, thus creating an error as if the tilt of the optical disk were measured.